Owing to the difficulty of their industrial synthesis, dialkylphosphinic acids and their derivatives have been used in industrial areas of application to only a relatively small extent.
Specific secondary phosphines obtained by addition of branched or cyclic olefins to phosphine, after their oxidation to dialkylphosphinic acids, are used as selective extraction media for cobalt/nickel separation.
Mixtures of perfluoroalkylphosphinic acids and perfluoroalkylphosphonic acids are used as specific extraction media. They originate from the reaction of perfluoroalkyl iodides with elemental phosphorus after hydrolysis and subsequent oxidation (DE-A-2 233 941).
To prepare dialkylphosphinic acid derivatives, such as the total herbicide phosphinotricin, a complex synthesis route via the hydrolysis of methyldichlorophosphine to give methylphosphonous acid and subsequent esterification of the abovementioned acid to give a phosphonous ester must be selected, before the second phosphorus-carbon bond can be made under free-radical induction.
The use of dialkylphosphinic acid derivatives as flame retardants for polyesters (poly(ethylene terephthalate) and poly(butylene terephthalate)) is described (EP 0 699 708 A1).
The end products are synthesized in a complex manner using methyldichlorophosphine as organophosphorus starting material, the hydrolysis product methylphosphonous acid and the ester of this acid having to be prepared and isolated as intermediates.
To prepare dialkylphosphinic acids or derivatives thereof, such alkylphosphonous esters can be alkylated by .alpha.-olefins at high temperatures under free-radical catalysis conditions. In the case of a reaction of alkylphosphonous acids under the same conditions, only the disproportionation products alkylphosphines and alkylphosphonic acids are obtained, however, whereas under mild conditions no reaction is observed.
U.S. Pat. No. 4,632,741 A1 describes a process for preparing mixtures of salts of alkylphosphonous and dialkylphosphinic acids, by reacting an olefin with a salt of hypophosphorous acid in the presence of a photoinitiator, using UV light.
U.S. Pat. No. 4,590,014 A1 describes a similar process in which the olefin is reacted with the alkali metal salt of hypophosphorous acid in the presence of a free-radical source (peroxide compound). However, the reaction finishes in this case as early as the stage of the alkylphosphonous acid.
According to Martinez et al. [Afinidad 53, 404 (1996)], in the above-described reaction, the yield of dialkylphosphinic acid is increased if said reaction is carried out in the presence of large amounts of sulfuric acid.
However, the above-described processes lead overall to unsatisfactory yields and in virtually all cases, in addition, to a mixture of reaction products, which must be worked up laboriously.